1. Field of the Invention
This invention relates to a wiring member having a conductive pattern formed on a substrate such as a high-density wiring board indispensable for the high-density packaging of electronic instruments.
2. Description of the Related Art
Concomitant with the trend in recent years to make electronic instruments portable and multifunctional, electronic instruments are now increasingly miniaturized. In particular, in the case of a functionally enhanced electronic instrument such as a portable telephone and a wearable computer, the employment of a high-density package is indispensable for enabling multifunctional electronic circuits to be accommodated in a housing of small capacity. In order to realize such a high-density package, it is essential to miniaturize the electronic components and at the same time, to provide a low-cost wiring board having a fine wiring pattern.
The wiring patterns formed on the surface of a wiring board have generally been formed through the etching of copper foil or metal plating using a resist pattern as a template. These methods necessitate complicated processes and expensive apparatuses, such as an exposure apparatus. Furthermore, since the printing of a wiring pattern is performed by using an exposure mask, an exposure mask is required to be newly prepared every time the design of a wiring pattern is altered, thus making it difficult to promptly cope with changes in designs of wiring patterns.
Under the circumstances, a method is now attracting much attention in which the wiring pattern is formed through the printing of a conductive paste on the surface of a substrate. In this printing method, an on-demand printing apparatus which is electrically connected to a computer is employed, so as to make it possible to directly form a desired wiring pattern based on the design data thereof, thereby making it possible to easily cope with the manufacture of small batches of different wiring patterns. Additionally, since the wiring pattern can be formed, in principle, through the printing and baking of a conductive paste, the steps for forming the wiring pattern are very simple.
As for the on-demand printing apparatus necessary, it would be advantageous to employ an ink jet printer because this makes it possible to form a very fine pattern. When printing a wiring pattern by using an ink jet printer, if the viscosity of a conductive paste is excessively high, it may become difficult to perform the printing of a wiring pattern with excellent resolution. Therefore, it is impossible to sufficiently increase the content of conductive fine particles in the conductive paste, thus limiting the content of conductive fine particles to about 10% by volume.
If the size of a droplet of ink is too large, it is also impossible to perform the printing of a wiring pattern with excellent resolution. Therefore, it is difficult to form a conductive pattern having a sufficient thickness by a single step of printing. Thus, if it is desired to secure a sufficient thickness of the conductive pattern, a plural number of printings are required to be repeatedly performed on the same position of the conductive pattern. Incidentally, if the subsequent printing is performed before the previous droplet of ink has completely dried, the printed pattern may be blurred. Therefore, the subsequent printing is required to be performed after the conductive pattern formed in the previous printing has dried sufficiently, thus raising the problem that the throughput of forming the conductive pattern would be greatly deteriorated.
Further, if a high-density packaging is to be realized, a plurality of wiring substrates each having a fine wiring formed thereon are required to be laminated to form a multi-layer wiring. On the occasion when a plurality of wiring substrates are stuck to each other, the portions to be stuck together are required to be capable of being deformed in conformity with the projected or recessed portions of the wiring substrates so as to absorb the projected or recessed portions of the wiring substrates. Accordingly, the wiring board commonly employed now is constructed in such a manner that a deformable adhesive layer is formed on the surface of a rigid base film or that a semi-cured resin is impregnated into a glass cloth or porous sheet. In the case of the former wiring board however, owing to a difference in characteristics between the base film and the adhesive layer, there is high possibility of generating peeling between the layers or warpage of wiring board. In the case of the latter wiring board on the other hand, although it is possible to realize excellent adhesion between layers as all layers of the wiring board are cured as an integral body, it would be impossible to prevent the entire body of the wiring board having wirings formed thereon from being once softened, thereby permitting the sinking or dislocation of wiring to easily take place. Once the wiring is permitted to sink or dislocate, the thickness of the insulating layer interposed between wirings is caused to change, resulting in the fluctuation of the properties of wiring such as impedance thereof. In particular, when the wiring is designed for high-frequency applications, the sinking or dislocation of wiring would lead to the problem that it may become more difficult to execute the matching of impedance.
Another method of manufacturing a multi-layer wiring board has been proposed by the present inventors for instance, wherein a plurality of sheet-like porous substrates each having wirings buried therein are laminated, to form a multi-layer wiring board. According to this method, since the projected and recessed portions originating from the provision of wirings can be suppressed to some extent, as the wirings are completely buried in the porous substrates, it is possible to relatively minimize the generation of the sinking or dislocation of wiring. However, due to the provision of the wirings buried in this manner, an inter-penetrating structure is formed between the porous substrate and the conductive material. As a result, it is more likely that these buried wirings would become relatively high in electric resistance as compared with the wirings which are not buried. It may be conceivable in this case that, in order to minimize the electric resistance, part of the wirings are formed on the outer surface of a porous substrate with the residual portions of the wirings being buried in the porous substrate. However, such a structure would lead to an increase in magnitude of the recessed/projected portions due to such a wiring structure, raising the problem that the sinking or dislocation of wiring may be permitted to easily generate.
As explained above, if the wiring pattern is to be formed through the printing of a conductive paste on the surface of substrate, the printing would be required to be repeatedly performed a plurality of times on the same location of the conductive pattern so as to secure a sufficient film thickness. In this case, if the next printing is performed before the droplet of ink applied to the previous printing is allowed to completely dry, the printed pattern may be blurred. Therefore, the next printing is required to be performed after the conductive pattern formed in the previous printing is allowed to sufficiently dry, thus raising the problem that the throughput of forming the conductive pattern would be greatly deteriorated.
Further, when it is desired to form a multi-layer wiring through the lamination of a plurality of wiring substrates, the sinking or dislocation of wiring may be permitted to easily generate on the occasion of sticking wiring substrates to each other, thus giving rise to fluctuations of properties of wirings.